1. Technical Field of the Invention.
This invention relates to printing presses, and more particularly to a densitometric scanner for sensing the density of an ink test strip printed on a page.
2. Related Prior Art.
It is well known that densitometric sensing devices have been used in connection with printing presses to measure the printed density of the ink on a zone-by-zone basis along a test strip usually printed along one edge of the sheet. Standards are set for the desired density in each zone and for each color, and the measured densities are compared against the standards. If one or more of the measured density values are outside the desired range set by the standards, adjustments are made to the press, typically to the ink supply device supplying the particular color ink to the zone in question, until the measured density is brought into the desired range. Sheets are scanned periodically to assure that the densities remain as desired and thus the printed product will be of high quality.
One such scanning device is shown in Ott EP-OS No. 149 424. As disclosed in Ott, an information code is printed adjacent the test strip for accurate location of the appropriate measurement position. This feature is expensive and has the disadvantage that even more space is required on the printed sheet for densitometric sensing, a consideration which further reduces its desirability.
A general disadvantage of devices which automatically scan a test strip for measuring its density is that the printed sheets must be so aligned with the measuring device that the strips which are printed on the sheet must be absolutely parallel to the travel of the sensing device. Thus, the sheets must be accurately aligned on the measuring table because if the strip and guide for the measuring head do not extend parallel to each other, the measuring head may move out of the range of the testing strip and produce faulty information. Indeed, the parallelism problem may arise as early as the make-up stage if the strip is not positioned correctly with respect to the copy. Further problems can be caused by distortion of the sheet in the printing process or by misfeeding of the sheet onto the measuring table.